I think this article sums is up well. https://bitcoinist.com/altcoins-coming-coinbase-says-ceo-brian-armstrong/

"Speculation is all that we currently have on what particular altcoins will be added to Coinbase. It is likely they will come from the top of the market capacity charts, which tend to be the most popular coins by demand. So in 2018 will we be seeing Ripple, NEO, IOTA, and Bitcoin Cash listed on Coinbase – or will they be selecting Dash, Monero, Nem, and Bitcoin Gold – or maybe all of the above?"